A technique of executing vector control on an alternating-current electric motor by using an inverter has been broadly used in an industrial field. This has been also broadly used for electric railroads. It has been known that when the above system is applied to a direct-current feeding railroad, electric oscillation occurs in an LC filter circuit comprising a reactor and a capacitor for absorbing higher harmonic waves which are disposed at the direct current side of an inverter, the voltage between both the terminals of the capacitor (capacitor voltage) is vibrated, and the control of the electric motor is destabilized. A damping control method to suppress this is disclosed in Non-patent document 1 and Non-patent document 2.
Non-patent document 1: Akira Kimura, et al., “Study on the Stabilization of Control System for Induction Motor Driving Rolling Stock”, Magazine D of articles of Institute of Electrical Engineers, Volume 110, No. 3, pp 291-300 in 1990
Non-patent document 2: keiichiro Rondo, et al., “Study on the Rotor Flux in Induction Motor in speed Sensorless Control for Railway Vehicle Traction”, Materials of Society for Semiconductor Power Conversion, SPC03-100, pp 69-74 in 2003.
According to both the Non-patent documents 1 and 2, the electric oscillation of the LC filter circuit is suppressed by adding a damping controller for detecting the voltage of a capacitor, extracting an oscillation component by a band pass filter (hereinafter referred to as BPF) to adjust the phase, multiplying the adjusted phase by a gain to obtain a damping operation amount, adding the damping operation amount to a slip frequency command (non-patent document 1) or a torque command (non-patent document 2).
The non-patent document 1 relates to an application to an electric motor control system to which slip frequency control is applied, and the Non-patent document 2 relates to an application to an electric motor control system to which vector control is applied.